A tunneling method for marking out drill hole patterns on the face of the rock by projecting the image of a slide provided with markings representing the positions of holes to be drilled on a reduced scale belongs to prior art is described in U.S. Pat. No. 3,431,049. This known method necessitates a separate direction determining means, a direction laser, for instance, as a reference in respect of the tunnel direction, as well as comparatively complicated measures for correcting the position of pattern points of a slide to correspond to correct positions of drill spots on the rock surface when projecting the slide image onto the rock face. This is due to the fact that the face of the rock, after a blast for instance, has varying shapes, which seldom or never constitute a substantially flat surface extending perpendicularly to the optical axis of the projector which projects the slide onto the rock surface. FIG. 1 of the accompanying drawing shows this.